To accomplish this task do the following steps:

1) Use the FILE - OPEN menu option to select your 
   last term's grades file.
2) Change the grade(s) as desired.
3) Now use the FILE - OPEN menu option again to select
   this term's grades file.
4) For the affected class remove the activity that holds
   the previous term's final grades.
4) From the TERM menu choose IMPORT AVERAGES/ACTIVITIES 
   FROM THE PREVIOUS TERM.
5) You will be presented with a file open dialog.  Choose 
   last term's grades file.
6) Now select the class for which you altered grade(s) and
   click OK.
7) Wait a minute for the process to complete.  Now check to
   make sure the averages were indeed updated.